1. Field of the Invention
The present invention relates to a welding machine having a movable frame that is equipped with a cutting means for cutting a plate to be welded (hereafter, a plate to be welded is referred to as a web, when applicable) in a width direction, and a welding means for welding cut portions of two cut plates obtained from the web, and that is elongated in the width direction of the web, so that the cutting and welding are performed while the movable frame is moved in the width direction of the web. In particular, the present invention relates to a plate width center alignment method and a plate width center alignment apparatus each for aligning plate width centers of the two cut plates with each other prior to the welding, which is employed by or included in a welding machine for welding cut portions of the two cut plates (obtained by cutting the web) to each other while overlapping or abutting the two cut plates to each other.
2. Description of the Related Art
FIG. 10 is a schematic view showing a plate width center alignment apparatus for a welding machine, which is disclosed in Japanese Patent Application Laid-open No. Sho 61-132269. Reference numeral 2 in FIG. 10 designates an anterior plate which is cut off from a web (a plate to be welded) consecutively supplied, and reference numeral 3 designates a posterior plate left as the web at the supply side. Reference numeral 55 designates a clamp of a positioning means 68 for clamping the anterior plate 2 to move the same, and reference numeral 66 designates a clamp of a positioning means 68 for clamping the posterior plate 3 to move the same.
This positioning means 68 is controlled by a not-shown control means. That is, the positioning means 68 is driven on the basis of correction values which are obtained by the control means through arithmetic processing on detected values sent from four plate side edge detectors (four optical plate side edge detectors 51a, 51b, 51c and 51d in this example) disposed independently to detect respective plate width ends, i.e. plate side edges, of the anterior and posterior plates 2 and 3.
The optical plate side edge detectors 51a, 51b, 51c and 51d mentioned above require a detection driving means for driving these detectors to detect the subjects to be detected, i.e. the plate side edges. In this example, the detection driving means is constructed by pulse encoders 54a and 54b, pinions 53a and 53b, support members 52a and 52b elongated from the pinions to support the plate side edge detectors 51a, 51b, 51c and 51d, and so on.
FIG. 11 shows a principle of the arithmetic processing for obtaining a deviation amount between the plate width central lines, which is executed by the controller. By detecting the positions of the respective plate side edges of the anterior and posterior plates 2 and 3, the control means performs the arithmetic processing to obtain distances A, B, C and D from the plate side edges to imaginary lines L1 and L2 parallel to plate width center lines used as references, and calculate a deviation amount .theta. between the plate width centers of the anterior and posterior plates 2 and 3, on the basis of which the plate width centers of the plates 2 and 3 are aligned to each other by the clamps 55 and 66 of the positioning means 68.
The conventional apparatus requires the detection driving means for driving the plate side edge detectors 51a, 51b, 51c and 51d for detection as described above. The installation of the detection driving means onto the welding machine makes the entire welding machine complicated and bulky, and the maintenance work difficult.
It is ideal that the plate side edges are detected from positions close to a cutting-welding position, i.e. an actual welding position, but the plate side edge detectors having the detection driving means occupy a large area to make it difficult to secure the installation space. Under this circumstance, the detection of the plate side edges must be carried out from a position far away from the cutting-welding position. The detection of the plate side edges from the far-away position encounters problems in that the detection is unreliable, for instance, in a case where the plate to be welded, i.e. the anterior plate or the posterior plate, is warped, to make is difficult to accurately align the plate width centers, and that the detection motion of the plate side edge detectors and thus drive control for the detection driving means is required independently in the sequence of the welding machine to elongate the entire operation time of the welding machine.